Introduction:  In  this  study,  we  utilized  the  MCNP4C  Monte  Carlo  code  to  quantitatively  evaluate  the 
influence of crystal size and material on intercrystal scatter and parallax effects.  
Materials and Methods:  For each of the 5 selected crystals (BGO, LSO, LYSO, LuAP, GSO), transport of 
511  keV  photons  originating  from  a  point  source  and  incident  on  the  central  crystal  of  a  block  detector 
consisting of a 13 × 13 array of 4 mm × 4 mm × 20 mm  crystals was simulated. Functions such as position 
detection accuracy (PDA) and point spread function (PSF) in different crystals were considered by changing 
the parameters such as crystal thickness, crystal pixel size and gamma ray angle of incidence. The code was 
validated against the simulated data published by Shao et al.  
Results:  The  results  of  the  simulations  and  their  analysis  showed  that  increasing  the  crystal  pixel  size 
increased  the  PDA  and  decreased  the  ICS  but  increasing  the  crystal  thickness  decreased  the  PDA  and 
increased the ICS. 
Discussion and Conclusion: By considering the attenuation properties of 511 keV photons, we conclude that 
BGO  is  the  most  appropriate  crystal  for  reducing  the  intercrystal  scattering  phenomenon.  However,  the 
higher light yield and lower decay time of LSO and LYSO makes them the most appropriate crystals for use 
in  PET  systems.  The  obtained  functions  for  intercrystal  scattering  can  be  used  for  recovering  spatial 
resolution and improving image quality in the image reconstruction process.